1. Field of the Invention
This invention is directed to a method of scoring a sheet of brittle material, and in particular, a method for scoring a glass sheet that minimizes uncontrolled crack propagation.
2. Technical Background
Mechanical tools are conventionally used for glass scoring. Alternatively, a laser beam, such as a CO2 laser beam at a wavelength of 10.6 μm, may be used. Typically, the glass is rapidly cooled immediately after heating by the laser to create transient tensile stress in the glass via thermal shock.
During laser scoring, a median crack (i.e. partial vent) is created that extends into the body of the glass. To create the vent, a small initial flaw is formed on the glass edge. The initial flaw may then be transformed into the vent and propagated by the laser beam. The initial flaw can be made by different methods. For example, the flaw may be introduced by a laser pulse or by a mechanical tool such as a scribe, scoring wheel, indenter, etc.
Heating of the glass with a laser beam creates a thermal gradient and a corresponding stress field that is responsible for the formation and propagation of the vent. If the heated glass is quickly quenched with a coolant the induced thermal stresses can be increased even further. However, propagation of the vent can be affected by the background stress in the glass (e.g. internal stress and stress that may be applied to the glass sheet from handling or holding the sheet during the scoring process). The background stress may become a significant factor when laser scoring is performed on just-formed glass as it cools, for example a sheet of glass immediately after it has been drawn from a molten mass. This may especially be the case during the removal of bulbous edges of the sheet (beads) that may have formed during the drawing of the sheet. The internal stress in the glass may be caused, for example, by differences in glass thickness produced during the forming process, and consequently by non-uniform glass cooling and a subsequent stress build-up concentrated in the beaded edge area. Additionally, stress can be applied to the glass by deformation (bending, twisting) of the sheet during glass handling by grippers or (and) suction cups used to handle the glass. If the background stress is high enough, it may cause uncontrolled propagation of the crack, which can start at the sheet edge where the initial flaw is located and deviate away from the desired scoring direction at the moment the laser beam reaches the sheet edge. More simply put, the thermal stress introduced by the laser beam can initiate a crack that propagates in an undesirable direction. Similarly, deviation of the vent from the prescribed score line can occur at the opposite end of the score, where the laser beam exits the opposing sheet edge.